Amps on the 12v rail

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pdonket

Limp Gawd
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Dec 15, 2006
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Alright, just wonder...when you guys say how many amps should be on a 12v rail, are you referring to 1 rail, or 4 (the sum of all of them) if its a quad railed PSU?

Just because I've seen people say "get 24 amps on the 12v rail" and looking into PSU's I haven't seen one with that many amps per single 12v rail.
 
pdonket said:
Alright, just wonder...when you guys say how many amps should be on a 12v rail, are you referring to 1 rail, or 4 (the sum of all of them) if its a quad railed PSU?

Just because I've seen people say "get 24 amps on the 12v rail" and looking into PSU's I haven't seen one with that many amps per single 12v rail.
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If it is multi rail it kind of depends on how the rails are confiugured. But in general people are referring to the sum wattage available on the 12v/12v to get the actual total 12v amperage as it is not a strict addition due to the way the 12v rails are fed. Some multi 12v rail units lack OCP so they are really in effect single rail units.
 
What specific model were you looking at? Different units behave differently.
 
The Seasonics are quad rail in name only. Put whatever fraction of the load you want on whatever rail, and it'll work.

The OCZ exceeds ATX spec for ripple. That's not a good thing. I'd stay away, personally.
 
Many P/S are now really only a single 12v rail internally. They will split the total amps up and some have a cuttoff if a rail tries to draw >20a. Some have even taken the cutoff out by Intel's blessing. P/S are different in how they show max amps/wattage. Some will show and absolute max wattage by robbing 3.3v and 5v. you have to find the correct wattage allowed for just the 12v and divide by 12.

ex. Seasonic S12EP 650w. It has a total of 52A listed on the 12v rail, which is 624w out of 650w. Of course that leaves the 3.3v and 5v rails nothing. So. 650w-170w(3.3/5v)=480w/12=40a. So you can support 40a max. Seasonic is good at max wattage but a cheaper quality P/S would need even more overhead.
 
Kabuto said:
ex. Seasonic S12EP 650w. It has a total of 52A listed on the 12v rail, which is 624w out of 650w. Of course that leaves the 3.3v and 5v rails nothing. So. 650w-170w(3.3/5v)=480w/12=40a. So you can support 40a max. Seasonic is good at max wattage but a cheaper quality P/S would need even more overhead.
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Not exactly. When Seasonic's labeling says it's PSU can go x amps on the 12v it can do x amps on the 12v. For instance the m12 700w can do 56 amps on the the 12v which is 672watts out of 700w. If you take a look at test 5 here it indeed does support 56 amps on the 12v.
 
Yes. If Seasonic prints it you can believe it since they build quality products. But while you could hit that max you do have to steal from the other rails. On the review he had to drop the load on the 3.3v/5v rails to get max wattage. Plugin my formula for the PSU label. 700w(total)-170w(3.3v/5v)-9.6w(-12v)-15w(+5vsb)=505.4w/12=42a max 12v without robbing other rails. Right about where his tests where and he had to start dropping 3.3v and 5v rail loads to go higher. Notice also, Seasonic took out the trip circuit as he was able to go >20a on 12V1.
 
Kabuto said:
Yes. If Seasonic prints it you can believe it since they build quality products. But while you could hit that max you do have to steal from the other rails. On the review he had to drop the load on the 3.3v/5v rails to get max wattage. Plugin my formula for the PSU label. 700w(total)-170w(3.3v/5v)-9.6w(-12v)-15w(+5vsb)=505.4w/12=42a max 12v without robbing other rails. Right about where his tests where and he had to start dropping 3.3v and 5v rail loads to go higher.
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You aren't "robbing" the other rails that is is how SMPS are designed to operate so your formula is not really relevant as if I were to max out the 3.3v and 5v I would be using 170w. SO look at what you said another way 700w-170w-9.6w-15w=505.4w. 505.4w/12v=42a means oh noes I am "robbing" the 12v.

Sorry but you simply are misrepresenting the facts to meet your contrived parameters.

Edit: Besides how do you plan to utilize the 5v and 3.3v to full capacity on a modern system at the smae time you are fulling utilizing the 12v?

Notice also, Seasonic took out the trip circuit as he was able to go >20a on 12V1.
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If you read above I also already mentioned the overcurrent protection not being present in all multi rail PSU's.
 
OK. I am confused by what you replied. My calculation gave the max wattage/amps on the 12v rail you could use and NOT steal wattage from the other rails. So the Seasonic can produce 42a and still provide max wattage to the other rails. Anything above that and you could run into problem as it takes wattage away from the other rails to the 12V rails. Am I wrong (not be sarcastic, just learning)? The test on it seems to verify that.

On the OCP I was agreeing and just pointing out to everyone and/or the OP.
 
Kabuto said:
OK. I am confused by what you replied. My calculation gave the max wattage/amps on the 12v rail you could use and NOT steal wattage from the other rails. So the Seasonic can produce 42a and still provide max wattage to the other rails. Anything above that and you could run into problem as it takes wattage away from the other rails to the 12V rails. Am I wrong (not be sarcastic, just learning)? The test on it seems to verify that.
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The point was it is not stealing wattage from the other rails it is how SMPS are designed to work. I pointed this out by the following:

You aren't "robbing" the other rails that is is how SMPS are designed to operate so your formula is not really relevant as if I were to max out the 3.3v and 5v I would be using 170w. So look at what you said another way 700w-170w-9.6w-15w=505.4w. 505.4w/12v=42a means oh noes I am "robbing" the 12v.
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Which simply takes your numbers and looks at them from the perspective of the 12v as opposed to the perspective of the 3.3v/5v (which are regulated together). In this perspective by your defintion the 3.3v/5v are stealing from the 12v. ;)
 
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